If you have players that become staff members you will need to create a new staff account for them. The system needs to have a unique identifier in order to know which version of the site they will have access to. Because these staff members will more than likely use the same email address for both accounts, the username will be the unique identifier.
For your staff members that want to continue to use one login to see both their player and staff information, they will need to use this new staff account to register for a league. Then you can merge their old player account with the new staff account. This will allow them to use a single login with their email address to access their staff and player information.